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Journal of
Materials Chemistry B
Materials for biology and medicine
www.rsc.org/MaterialsB
Volume 1 | Number 31 | 21 August 2013 | Pages 3719–3870
Themed issue: Carbon Bioelectronics
ISSN 2050-750X
FEATURE ARTICLE
Eric Daniel Głowacki et al.
Hydrogen-bonds in molecular solids – from biological systems to organic
electronics
Journal of
Materials Chemistry B
FEATURE ARTICLE
Cite this: J. Mater. Chem. B, 2013, 1,
3742
Hydrogen-bonds in molecular solids – from biological
systems to organic electronics
Eric Daniel G1owacki,*a Mihai Irimia-Vladu,abc Siegfried Bauerb
and Niyazi Serdar Sariciftcia
Hydrogen-bonding (H-bonding) is a relatively strong, highly directional, and specific noncovalent
interaction present in many organic molecules, and notably is responsible for supramolecular ordering
in biological systems. The H-bonding interactions play a role in many organic electrically conducting
materials – in particular in those related to biology, e.g. melanin and indigo. This article aims to
highlight recent work on application of nature-inspired H-bonded organic molecules in organic
Received 10th February 2013
Accepted 27th February 2013
electronic devices. Three topics are covered in this brief review: (1) electrical and ionic conduction in
natural H-bonded systems, (2) semiconducting properties of H-bonded organic pigments, and (3)
DOI: 10.1039/c3tb20193g
exploitation of H-bonding for supramolecular assembly of organic conductors. H-bonding interactions
are ubiquitous in biology, thus making the study of H-bonded organic semiconductors highly pertinent
www.rsc.org/MaterialsB
where interfacing of electronics with biological systems is desired.
The hydrogen bond
Linus Pauling wrote in 1940, “It has been recognized in recent
years that under certain conditions an atom of hydrogen is
attracted by rather strong forces to two atoms, instead of only
one, so that it may be considered to be acting as a bond between
them.”1 In the 70 years since Pauling, many noncovalent
interactions in structural organic chemistry have been classied
as H-bonds, creating a difficulty in furnishing a satisfying allencompassing denition of what constitutes an H-bond. A
useful explanation is perhaps provided by Peter Atkins in 1989,
“A hydrogen bond is a link formed by a hydrogen atom lying
between two strongly electronegative atoms”.2 Many types of
H-bonds exist, spanning a wide range of bond energies and
geometries. A hydrogen atom can be shared between an
“H-bond donor” and an “H-bond acceptor”, a molecule with
electron lone pairs. H-bond donor character increases with
acidity while H-bond acceptor character increases with basicity.
Such an interaction can involve an interplay of multiple H-bond
acceptors interacting with a single donor.3–5 A schematic with
the major types of H-bond geometries is shown in the centre of
Fig. 1, together with examples where H-bonds play an important
role – in biomolecules, dyes and pigments, as well as in ionic
conductors and organic semiconductors. This review aims at
elucidating the role of H-bonds in these material classes,
providing links between the seemingly unrelated dyes and
a
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler
University, Linz, Austria. E-mail: [email protected]
b
c
So Matter Physics, Johannes Kepler University, Linz, Austria
Joanneum Research Forschungsgesellscha mbH, Weiz, Austria
3742 | J. Mater. Chem. B, 2013, 1, 3742–3753
pigments and organic semiconductors, and suggesting new
research avenues to merge ionic and electronic devices.
Hydrogen bonds in biological systems
Though H-bonds are typically weaker than most covalent
bonds, with bond energies of the order of 5–65 kcal mol1, they
are usually signicantly stronger than van der Waals interactions, which are less than 5 kcal mol1.4–8 In biological
systems the role of H-bonds is ubiquitous. Biochemistry exists
in an aqueous environment, where H-bonding interactions are
responsible for many of the properties of water, such as high
boiling point, dielectric constant, and surface tension. Perhaps
the most familiar H-bonded biomolecule in biology is DNA –
where base pairs are held together by highly specic H-bonding
between amine proton donors (–NH2 or –NH) and carbonyl
group (]O) acceptors. H-bonding is the most signicant noncovalent interaction in most polypeptides, i.e. proteins. The two
most common protein secondary structures – alpha helices and
beta pleated sheets – are both held together by H-bonding
between amino acids on adjacent chains, likewise caused by an
amine hydrogen–carbonyl interaction of the type –NH/O].
The antiparallel beta sheet, one of the biologically important
structures this type of bonding leads to is illustrated schematically in Fig. 1. Amine–carbonyl bonds are the most frequently
encountered but not the only important category of H-bonding
in biomolecules: H-bonds between –OH groups comprise the
interchain interactions between polysaccharides such as cellulose and chitin and are thus responsible for the robust
mechanical properties of these structural biopolymers.
This journal is ª The Royal Society of Chemistry 2013
Feature Article
Journal of Materials Chemistry B
Fig. 1 Summary of the role of hydrogen-bonding in biomaterials, dyes and pigments, ionic conductors, and organic semiconductors. The central box shows a
schematic summary of the H-bond: an interaction between a proton donor and a proton acceptor. H-bonding is an extremely important aspect in the structural organic
chemistry of biomaterials such as proteins, DNA, and sugars, as well as in dyes and pigments of natural origin. Connections between the four categories, representing
the overlap of applicability of materials, are shown with black lines. The image of an electronic ion pump in the portion of ionic conductors is reproduced with
permission from ref. 9.
Since H-bonding interactions are so common in the natural
world it is worthwhile to consider what role they may play in
organic conducting and semiconducting materials. At present,
the eld of organic electronics has evolved from the scientic
laboratory into several mature commercial technologies, such
as xerography10 and organic light-emitting diodes;11 with
organic photovoltaics12 and organic transistor-based circuits13
poised to enter the market in the near future. Since early work in
the 1960s where it was discovered that conjugated aromatic
compounds exhibited semiconducting properties,14–16 research
in organic materials for electronics has been focused on materials with extensive p-conjugation as well as p–p stacking
between adjacent molecules.17 The rst approach of using an
extensive p-conjugated system yielded conducting conjugated
polymers like polyaniline and polyacetylene – 1D metallic conducting systems for which the Nobel Prize in chemistry was
awarded in 2000.18–20 The ultimate materials science realization
of conduction through a 2D p-conjugated system is graphene,21
an achievement likewise recognized with a Nobel Prize in 2007.
Difficulties in fabricating useful devices based on quasi 1D and
2D systems have led to the overall predominance of the second
approach: involving conduction through the p–p stacking of
adjacent overlapping conjugated molecules. It is exactly on this
second concept that organic semiconductor devices like lightemitting diodes and solar cells are based.
This journal is ª The Royal Society of Chemistry 2013
Since H-bonding interactions are so prevalent in nature, and
have been exploited extensively in the eld of organic crystal
engineering for commercial colorant applications, it is
surprising that exploiting H-bonding as a molecular design tool
for obtaining organic semiconducting and conducing materials
has remained largely unexplored. Nevertheless three distinct
research avenues into utilizing H-bonding for organic electronic
devices are identied, and these are the following: (1) consideration of conduction mechanisms in H-bonded biological
materials like DNA, (2) H-bonded organic pigments, used
extensively in industrial coloristic applications, and (3) supramolecular H-bond mediated self-assembly of electronically
conducting moieties. Because of the important role of
H-bonding interactions in biology, organic electronic materials
featuring H-bonding could be of substantial interest in
emerging organic electronic applications,22–24 where interfacing
with biological/biomedical systems is desired, such as biosensors, analytical/diagnostic devices, drug delivery systems, and
interfaces with neural tissues.
Conduction in H-bonded biomaterials
Since the earliest days of organic electronic conductor research,
H-bonded natural-origin molecules have been the centre of
attention. Questions of conductivity in double-stranded DNA
J. Mater. Chem. B, 2013, 1, 3742–3753 | 3743
Journal of Materials Chemistry B
and RNA and pigmentary melanin have been a topic of fundamental experimental interest. DNA is composed of a deoxyribose sugar chain backbone with planar nucleobases bringing
together two complementary DNA strands via H-bonding. The
four nucleobases are shown in Fig. 2a. Eley and Spivey observed
in 1962 that the H-bonded nucleobase pairs within the double
helix of the DNA p–p stack with a distance of 3.4 Å, a value
similar to graphite. From temperature-dependent DC conductivity measurements of dry DNA, they concluded that DNA was a
semiconductor with conduction arising from thermally excited
electrons traversing the stacked base pairs along the chain; and
a bandgap of 2.4 eV.25 Using photoinduced electron transfer
experiments, in the 1990s and early 2000s several researchers
showed that electron transfer can proceed efficiently over
distances of several nanometers along the DNA strand.26,27
These values are considerably higher than the value of charge
transfer efficiency predicted according to Marcus theory. It was
Fig. 2 (a) DNA base pairs held together by complementary H-bonds. (b)
Experimental technique of Barton et al. whereby fluorescence quenching via
charge transfer of dyes intercalated at different positions along a DNA strand is
used to measure electronic conduction through the base pair stack. Reproduced
with permission from ref. 28. (c) Examples of contacting individual DNA strands
for electrical measurements. These experiments yielded controversial results.
Reproduced from ref. 27. (d) Multiple researchers have concluded that conduction proceeds through stacked G–C pairs, however A–T pairs are barriers.
Reproduced from ref. 27.
3744 | J. Mater. Chem. B, 2013, 1, 3742–3753
Feature Article
found by varying the nucleobase sequence that the efficiency of
charge transfer (via hole transport) was dependent on how well
nucleic base molecules were stacked. Several groups have
shown that the thymine–adenine sequence has the poorest
overlap and thus impedes conduction, while guanine–cytosine
or guanine–guanine stacking was superior. In a series of papers
by Barton et al.28,29 electron transfer reactions were probed by
intercalating octahedral metal complexes at various points
along a DNA double strand and measuring uorescence
quenching. This technique is shown schematically in Fig. 2b.
These studies concluded that efficient intrastrand electron
transport occurs along stacked base pairs.
Direct electrical interrogation of conductivity of DNA strands
requires manipulation of strands to lie between two electrodes.
Dekker et al. in 2000 used electrostatic trapping techniques to
position single strands (10 nm in length) of cytosine–guanine
containing DNA between metal nanoelectrodes.30 A similar
technique was used by Fink and Schönenberger to measure
strands of the length 1 mm.31 From their temperature-dependent measurements in air and in a vacuum both groups
concluded that DNA was a wide-bandgap semiconductor. Other
researchers have used solution-deposition methods to stretch
strands of DNA across micropatterned electrodes and have also
conrmed the semiconducting behaviour of DNA.32 Extensive
theoretical work on polaronic transport in DNA has been
reported by Conwell and coworkers.33,34 Nevertheless, other
studies using direct electrical contact techniques have indicated
that DNA is an electrical insulator at length scales greater than
40 nm.35 It has been suggested that residual salts contribute to
conduction and that conductivity derives from an adsorbed
water layer on the DNA and that conduction is absent along the
p–p stacked backbone of DNA at longer length-scales.36
Signicantly, “bulk” processed DNA is an insulator, and in fact
has been exploited as such to provide a gate dielectric for
organic eld-effect transistors.37,38 Vacuum-evaporated thinlms of individual base-pair molecules, such as adenine and
guanine, were determined to be good insulators as well.39
To-date, no electronic device relying on the conductive properties of DNA has been demonstrated, although conducting
polymer–DNA composites exploiting the interesting selfassembly properties of DNA show some more promise.40 In
conclusion, the existence of electrical conductivity along the
H-bonded and p–p stacked backbone remains controversial, at
least for device-relevant length scales.
Eumelanin, the pigment responsible for human skin, hair,
and eye color, has attracted signicant attention for potential
electrical applications. Eumelanin is one of the many melanin
pigments which are ubiquitous in nature. Another well-known
melanin is the octopus ink, for example. Melanins are
composed of a complex mixture of crosslinked oligomeric and
polymeric dihydroxyindoles and dihydroxyindole carboxylic
acids (Fig. 3a).
These molecules afford extensive H-bonding interactions.
McGinness et al. reported in 1974 that melanin behaves as an
amorphous semiconductor threshold switch.42 This pioneering
organic electronic device is shown in Fig. 3b, along with its
current–voltage characteristics. This device acted as a threshold
This journal is ª The Royal Society of Chemistry 2013
Feature Article
Journal of Materials Chemistry B
changes in conductivity were rationalized by accepting that the
effective dielectric constant increased in the presence of water.
This model oen ts well to experimental data.47 Recent work
relying on measurements where differing surface areas of the
melanin sample are exposed to water vapour (Fig. 3c) have
shown that the MDAS model is most likely incorrect and that in
fact hydration-dependent ionic conductivity plays a dominant
role in melanin conductivity.41 The existence of hybrid ionic–
electronic conductivity in melanin suggests that it is an
intriguing candidate for bioelectronics interfacing. Indeed
recent studies have shown the compatibility of melanin with
neuron cells.48
Ionic conductors
Since biological systems rely on transport of ions, e.g. H+, Ca2+,
Na+, and acetylcholine, direct interfacing with biology requires
transduction of ionic signals into electronic ones. Signicant
progress to this end has been made in the past decade, generating a eld oen called Iontronics.49 Solid-state devices based
on ion-conducting/electron conducting polymers capable of
neural interfacing50–52 or delivery of neurotransmitters9,53,54
exemplify this effort. H-bonded biomaterials have also been
explored as part of this eld: polysaccharides such as cellulose
or chitin are classic examples of polymeric materials where
interchain H-bonding profoundly inuences mechanical properties. Though they are electrically insulating, polysaccharides
Fig. 3 (a) An example of the polymerized heterocycles that comprise melanin.
(b) I–V characteristics and photograph of the original melanin “threshold switch”
device fabricated by McGinness. (c) Relationship of conductivity to water uptake
in melanin, from ref. 41.
switch, switching to a high-conductivity state once a certain
threshold voltage was reached. This process was found to be
fully reversible. What remained mysterious, however, was the
role of hydration in electrical conductivity, as thoroughly drying
the device led to a disappearance of electrical conductivity. X-ray
structural studies reported in 1994 on one of the fundamental
structural units of melanin, 5,6-indolequinone, showed that
these units closely p–p stack with an interplanar distance of
3.4 Å.43 Device-quality thin lms of melanin with DC conductivities comparable to amorphous silicon, as well as photoconductivity response, have been demonstrated. These lms can be
prepared on a variety of substrates by solution processing from
aqueous solutions.44,45 Read-only memory devices have also
been fabricated from melanin.46 Reconciling various experimental results showing hydration-dependent conductivity and
photoconductivity has led to a controversy about the true nature
of conduction in melanin. Most researchers applied a Mott–
Davis amorphous semiconductor model (MDAS), where
This journal is ª The Royal Society of Chemistry 2013
Fig. 4 (a) Bioprotonic transistor based on chitosan as a proton-conducting
material. (b) Illustration of the Grotthus-like mechanism of proton conduction
along the chitosan polysaccharide. Reproduced with permission from ref. 56.
J. Mater. Chem. B, 2013, 1, 3742–3753 | 3745
Journal of Materials Chemistry B
can be effective ionic and protonic conductors. Zhong et al.
showed in 2011 that solid-state electric eld-effect devices can
be realized using the polysaccharide chitosan as a protontransporting matrix.55,56 A schematic of this device is shown in
Fig. 4a. Highly doped silicon and SiO2 serves as the gate electrode and gate dielectric, respectively. Palladium hydride
source–drain “protodes” serve as proton-transparent contacts.
By applying a gate voltage, protons are either induced or
depleted in the proton-conducting chitosan channel, thus
providing an electrical modulation of protonic current. Transport of protons along the H-bonded chitosan polymer is illustrated in Fig. 4b. This work represents a successful
demonstration of controlling protonic current in a solid-state
device based on H-bonded biomaterials.57
H-bonded pigments
Organic pigments are colorants that by denition are insoluble
in the media they are applied in. Various paints and inks consist
of pigment particles with size ranging from nanometers to
several microns mixed with dispersing agents. In all cases,
industrial organic pigments owe their poor solubility to strong
intermolecular interactions, which are oen an interplay of
strong p-stacking and H-bonding.58 Diketopyrrolopyrroles
(DPPs), quinacridones, perylene diimides (PDIs), and indigos
are four of the major classes of industrial colorants today. All of
these pigment classes feature H-bonding between –NH hydrogens (donor) and carbonyl groups (acceptor), reminiscent of the
same type of H-bonding interactions present in proteins.
The interplay of H-bonding and p-stacking results in high
lattice energies. This fact is reected in the remarkable thermal
stability of these pigments; they are known to sublime at very
high temperatures (e.g. >500 C for quinacridone). To the best of
our knowledge no H-bonded pigments have been reported to
have melting points. Though these pigments are very wellknown for decades, they have received nearly no attention as
potential organic semiconductors. The reason for this is that
these molecules contain carbonyl and amine groups in conjugated segments. Since under neutral pH conditions mesomeric
forms featuring enol or imine character are unfavourable,
carbonyl and amine groups are seen as interrupting p-conjugation. The prevailing notion among synthetic chemists is that
when designing organic semiconducting small molecules or
polymers, maximizing p-conjugation is crucial, and thus amine
or carbonyl functional groups are avoided.17,59 Recent demonstrations of high carrier mobility in H-bonded pigments containing such functional groups raise the question that perhaps
the ‘design guidelines’ for organic semiconductors could
require modication. H-bonded pigment molecules measured
in recent years as organic semiconductors are shown in Fig. 5,
along with their reported mobility data. The molecules shown
will be covered in the following section.
Feature Article
dibrominated derivative, tyrian purple, are the oldest and
probably best-known organic colorants of natural origin
(structures shown in Fig. 5). Indigos are dimers of indole
heterocycles, which originate from the amino acid tryptophan.
Indigo is formed in certain plants of the indigofera genus,60
while tyrian purple originates from shellsh.61 Recent work has
shown that indigo and tyrian purple, operate as semiconductors
in
organic
eld-effect
transistor
(OFET)
devices.62
Ambipolar transport of electrons and holes with a mobility of
1 102 cm2 V1 s1 was found for indigo.63 Interestingly, this
device demonstration consisted of entirely biodegradable
materials, including the shellac resin as a substrate. The
ambipolarity was correlated with an electrochemical behaviour
of reversible reduction and oxidation at relatively low potentials. Injection of holes and electrons from a single source–
drain contact material (in this case Au) was facile due to the low
bandgap of the material, 1.7 eV. Studies of the dibrominated
derivative tyrian purple demonstrated a higher mobility,
0.4 cm2 V1 s1 for both electrons and holes.64,65 The carrier
mobility in the indigos is correlated with their crystal structure.
In most indigos, individual molecules form hydrogen-bonds
between adjacent N–H/O] groups along one crystallographic
axis. Along a perpendicular axis, the indigo molecules interact
via p-stacking. This stacking is close and relatively cofacial
(compared with typical herringbone crystals like the acenes)
with an interplanar distance of 3.4 Å and the distance between
equivalent positions on neighbouring molecules of 3.6–5 Å –
this number can be also higher, depending on substituent
groups on the phenyl rings of indigo. The crystal structure of
tyrian purple, is shown in Fig. 6.
Intermolecular H-bonds are formed along one crystallographic axis (c-axis) whereby every indigo molecule is H-bonded
via single H-bonds to four neighbouring molecules. Meanwhile,
p–p stacking interactions exist along an axis perpendicular to
the H-bonding axis, in the case of indigo and tyrian purple the
b-axis. Thus despite the fact that indigos can be considered,
from the traditional perspective of conjugated organic semiconductors, to have minimal p-conjugation, their intermolecular p-stacking is apparently sufficient enough to support
charge transport with relatively high carrier mobilities, 0.01–
0.4 cm2 V1 s1. Signicantly, controlling the crystallographic
orientation of indigos during growth of semiconducting thin
lms is crucial for charge carrier mobility.
Since the b-axis is the p–p stacking axis, this axis must be
oriented in the direction of charge transport. For OFET devices,
this can be accomplished by modifying the gate dielectric with a
hydrophobic layer (such as tetratetracontane62,66 or polyethylene65) so that the indigo molecules grow in a “standing-up”
orientation by virtue of the favourable dispersion forces
between the hydrophobic phenyl rings on indigo and the
aliphatic substrate.
Quinacridone and DPP
Indigo pigments
An example of H-bonded molecules that ‘break the rules’ for
obtaining semiconducting behaviour is indigo. Indigo and its
3746 | J. Mater. Chem. B, 2013, 1, 3742–3753
Linear-fused ring H-bonded molecules can be regarded as an
interesting case study, as they are direct analogues of the wellknown acenes, e.g. pentacene. Epindolidione and quinacridone
This journal is ª The Royal Society of Chemistry 2013
Feature Article
Fig. 5
Journal of Materials Chemistry B
Summary of the molecular structures and field-effect mobility measured for the H-bonded pigments covered in this section.
Fig. 6 (a) View down the b-axis, or p-stacking axis, of tyrian purple. (b) Crystal
packing in tyrian purple, showing the interplanar, atom-to-atom, and H-bond
distances. Each molecule is H-bonded to 4 neighbours.
This journal is ª The Royal Society of Chemistry 2013
are the 4- and 5-ring analogues of tetracene and pentacene,
respectively (Fig. 7a).
In a recent study,67 it was shown in identically fabricated
OFET devices that epindolidione afforded a signicantly higher
hole mobility than tetracene, 1.5 cm2 V1 s1 vs. 0.1 cm2 V1
s1, and that the epindolidione-based devices were highly stable
in air for over a hundred days, while tetracene degraded
immediately. A comparison of mobility and on/off ratio
stability over time for tetracene and epindolidione is shown in
Fig. 7b. Quinacridone was found to have a hole mobility of
0.2 cm2 V1 s1, lower than pentacene, however, the quinacridone devices were highly stable under operation for 140+ days
in air while pentacene was far less stable (Fig. 7c). Quinacridone
also could support ambipolar transport with appropriate
source–drain contact metals. Epindolidione and quinacridone
are established industrial pigments, used for high-performance
outdoor paints and in inkjet toner formulations. Exploiting a
class of materials already mass-produced and cheaply available
could enable low-cost large-area organic electronics.
In the case of indigo it has been found that the surface
properties of the substrate material are critical for molecular
orientation and thus good charge transport. The crucial role of
surface properties appears to extend to other H-bonded
pigments as well. A 2008 study of OFET devices using quinacridone and DPP using SiO2 as the gate insulator onto which the
molecules were processed found mobilities not exceeding 1 105 cm2 V1 s1.68 Another study from a different group in
2009 found a mobility of 4 104 cm2 V1 s1 in quinacridone
and also on SiO2.69 The use of an aliphatic passivation layer on
the gate dielectric in the 2013 study67 of quinacridone signicantly increased observed mobilities to 0.2 cm2 V1 s1. Details
concerning the role of the substrate in the growth of lms of
such materials, however, remain ambiguous.
The crystal structure of quinacridone is known to adopt one
of the four polymorphs, two of which, aII and g, feature 4 single
J. Mater. Chem. B, 2013, 1, 3742–3753 | 3747
Journal of Materials Chemistry B
Feature Article
Fig. 8 (a) Crystal packing in b-quinacridone, which consists of infinite linear
chains of H-bonded molecules. (b) Quinacridone H-bonding in the linear-chain
polymorphs. (c) Schematic illustration of the “bricks-in-a-wall” crystal packing
present in many H-bonded pigments, whereby linear chains of H-bonding
propagate along one direction, while there is p–p stacking perpendicular to the
H-bonding direction. Reproduced with permission from ref. 71.
Fig. 7 (a) The acenes and their structural H-bonded analogues, epindolidione
and quinacridone, reported in ref. 67. (b) Comparison of operational stability in air
of tetracene and epindolidione. (c) Comparison of operational stability in air of
pentacene and quinacridone. Reproduced with permission from ref. 67.
H-bonds to 4 neighbours (like the indigos). The other two
polymorphs, aI and b, are markedly different: in these, quinacridone molecules form double H-bonds to two neighbours,
thus forming innite linear chains. In the perpendicular
direction, p–p stacking exists.70–73 This arrangement, known as
the “bricks in a wall” or “linear-chain” is shown in Fig. 8.
Evaporation of quinacridone onto substrates held at low
temperatures (25–70 C) yields the linear-chain polymorph
(probably b) in both OFET67 and diode74 devices. In these ‘linear
chain’ polymorphs, the p–p interplanar spacing is 3.4 Å. The
“bricks in a wall” arrangement, featuring innite H-bonded
molecular chains, present in aI and b quinacridone, 2,9-dimethylquinacridone,75 and most reported DPP pigments,76 is
indeed very different from the herringbone structures present
in many molecular semiconductors such as the acenes.
3748 | J. Mater. Chem. B, 2013, 1, 3742–3753
It should be noted here that much effort77 in obtaining superior
organic semiconductors has been focused on substituting
molecular semiconductors such as to overcome the herringbone crystal formation and enforce a more cofacial p–p intermolecular interaction, like the one present in the “bricks in a
wall” crystal structures. It seems that relying on H-bonding
intermolecular interactions can be a useful design technique
for achieving such desired crystal structures.
A notable property of all the H-bonded pigment-forming
small molecules – including quinacridone, indigo, DPP, and
epindolidione – is that their optical absorption undergoes a
substantial (50–100 nm) bathochromic shi from a dilute
solution to the solid state.78,79 Blocking H-bonding, by for
example methylating the –NH groups in indigo79,80 or quinacridone,81 eliminates this effect, and solution spectra closely
resemble solid-state spectra. These observations signal the
involvement of strong intermolecular electronic coupling that is
caused by intermolecular H-bonding. This phenomenon has
been studied in detail for g quinacridone and several DPP
pigments.76 What has been found is that the large bathochromic shis arise from an interplay of two effects: (1) the
sharing of a proton between adjacent molecules participating in
H-bonding changes the local electronic structure of the individual molecules. From a chemical perspective of mesomerism,
the interaction of N–H and ]O groups of neighbouring molecules can be seen to strengthen the resonance contribution of
the imminium and enolate mesomers, essentially enhancing
the p-conjugation of the individual chromophores.
This journal is ª The Royal Society of Chemistry 2013
Feature Article
This phenomenon of “solid state mesomerism” has been
reviewed recently by Lincke.70,71 (2) H-bonding enhances the
electronic coupling between molecules, i.e. there is substantial
delocalization of electronic wavefunctions between neighbouring molecules. Studies of polarized optical reectivity on g
quinacridone single crystals showed that the low energy transition that appears in the solid state is oriented along the
H-bonding axis.81 The results were interpreted in the following
way: H-bonding in quinacridone leads to head-to-tail arrangement of the transition dipoles of neighbouring molecules,
shiing the absorption band towards longer wavelengths. The
photoluminescence of quinacridone in the solid state is more
red shied and broad compared with solution spectra, a
phenomenon that has been interpreted as arising from excimer
emission.82–85 It was recently found that metal–insulator–metal
diodes with quinacridone as the active material showed a
pronounced photovoltaic effect and achieved external quantum
efficiencies of 10%, a number three orders of magnitude
higher than the related molecule pentacene, as well as most
other organic conjugated materials in the absence of a donor–
acceptor junction.74 It was thus proposed that excimeric states
forming in quinacridone lms can be easily polarized into free
charge carriers. Older work, aimed at xerographic applications,
has shown that similar efficient photogeneration occurs in DPP
pigments as well.86 Dissociative excited states in such pigments,
thus, can possibly be an interesting research avenue for
photodetector and photovoltaic applications that differ from
the well-established donor–acceptor heterojunction concept.
DPPs have recently gained a great deal of attention as
building blocks for polymers and oligomers with some of the
highest mobility (in organic transistors) and power-conversion
efficiencies (in photovoltaics).87,88 Interestingly, in all of these
reports the –NH groups are substituted with solubilizing
aliphatic side groups and the potential of interchain H-bonding
has not been explored. Only recently, in 2012, it was shown that,
through the use of thermolabile protecting groups, DPP-based
H-bonded oligomers and polymers can be solution
processed and when deprotected, can support ambipolar
transport in its H-bonded form with a eld-effect mobility of
6 103 cm2 V1 s1 for both carriers in the H-bonded oligomers, and even in the 102 range for electrons in the case of
the polymer.89,90
Other H-bonded pigments
A few other pigment molecules featuring H-bonding have been
reported in organic eld-effect transistor applications. A littleknown pigment, 6H-pyrrolo[3,2-b:4,5-b0 ]bis[1,4]benzothiazine
(PBBTZ), shown in Fig. 5, rst synthesized in 1968, was utilized
by Zhu et al. to fabricate transistors with a hole mobility of
0.34 cm2 V1 s1 and exceptional stability in air for at least 8
weeks.91 It was found that blocking the intermolecular
H-bonding in PBBTZ by replacing the amine hydrogen with an
aliphatic side group resulted in a drop in mobility of more than
three orders of magnitude. The study concluded based on X-ray
diffraction and atomic-force microscopy evidence that
H-bonding was critical for providing a well-ordered crystalline
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Journal of Materials Chemistry B
structure supporting high mobilities. The authors followed up
this study by fabricating a single-crystal transistor based on
PBBTZ, obtaining a hole mobility of 3.6 cm2 V1 s1 and an
operational stability of at least one year.92
Another industrially important family of pigments is that of
perylene bisimides. They, in contrast to most of the pigments
discussed here, have been widely explored for organic devices as
effective electron-transporting materials in transistors and
photovoltaic diodes. This subject has been extensively
reviewed.93,94 Once again, however, the imide –NH groups in the
structure responsible for H-bonding with carbonyl ]O groups
on adjacent molecules are blocked with R groups in nearly every
reported study, to our knowledge. A recent exception was
reported in 2010, where a perchlorinated perylene
bisimide, Cl8-PTCDI, was found to have an electron mobility of
0.91 cm2 V1 s1, as well as stability in air due to a deep-lying
LUMO level.95 Though the molecule is highly twisted due to
steric effects of the chlorine atoms, the intermolecular
H-bonding enforces a “bricks in a wall” crystal structure with a
good intermolecular p–p overlap. Signicantly, it was found as
in the case of PBBTZ that interrupting H-bonding, this time by
replacing the imides with anhydrides, resulted in a three-orderof-magnitude decrease in eld-effect mobility.
The group of Miao has published results on N-heteroquinones, pentacene analogues with intermolecular Hbonding. These materials demonstrated an electron mobility of
0.1 cm2 V1 s1.96 This study was followed by a new set of acene
analogues with complementary H-bonding between adjacent
pyrazine and dihydropyrazine rings, affording a hole mobility
up to 0.7 cm2 V1 s1.97 These molecules also crystallize in a
bricks-in-a-wall geometry, with excellent cofacial p–p stacking.
Synthetic H-bonded supramolecular
semiconducting assemblies
It is clear from biomolecules such as proteins and DNA that
H-bonding is a highly specic tool for self-organizing organic
materials. There have been numerous studies on utilization of
H-bonding mediated self-organization in the eld of synthetic
materials science, however, relatively few in the eld of organic
electronics. A few illustrative examples will be highlighted here.
In the early days of organic semiconducting materials, it was
recognized that deviation from planarity of conjugated
segments leads to hypsochromic shis in optical absorption
and oen poor conductivity. It was found that certain Hbonding interactions introduced along a polymer chain could
be used to enforce planarity. Interaction between –OH groups
and the Schiff base (–C]N–) for example, can lead to substantial bathochromic shis in absorption of polyazomethine
polymers due to planarization of the polymer.98 Various
approaches involving H-bonding between adjacent chains can
also be exploited to planarize polymers.99,100
The next level of utilizing H-bonding is making conducting
supramolecular assemblies. H-bond mediated supramolecular
semiconducting systems have been demonstrated for eldeffect transistor and light-emitting diode applications. Schenning, Meijer, and coworkers in Eindhoven have done the most
J. Mater. Chem. B, 2013, 1, 3742–3753 | 3749
Journal of Materials Chemistry B
extensive work on this concept up-to-date. In one demonstration, H-bonding was utilized to drive the self-assembly of
hole-transporting oligo(p-phenylenevinylene)s and electrontransporting perylene bisimides (Fig. 9a).101 The resultant
supramolecular composite consisted of uniform rod-like
domains which supported ambipolar transport. These
H-bonded ambipolar dyads afforded a low mobility, 1 107 cm2 V1 s1. A mixture, however, of the two components
without H-bonding moieties yielded donor–acceptor complexes
with no eld-effect mobility at all. The H-bonding thus enforces
a supramolecular morphology supporting two independent
pathways for charge transport. In a subsequent report, white
light-emitting electroluminescent devices were achieved by
using an H-bonded supramolecular polymer consisting of red,
green, and blue emitters.102 Each of the chromophores was
functionalized with self-complementary quadruple H-bondforming ureidopyrimidinone units at both ends (Fig. 9b).
These molecules self-assemble into supramolecular polymers which can efficiently support resonant energy transfer
(Fig. 9c). At a certain mixing ratio, white uorescence in solution is observed (Fig. 9d and e), a fact that was exploited to
fabricate organic-light emitting diodes that demonstrated white
electroluminescence. The utilization of complementary
H-bonding afforded several advantages relative to mixing
unfunctionalized chromophore components: (1) no phase
separation is observed, (2) the H-bonded system provided
superior lm-forming properties, and (3) white-light emission is
signicantly more efficient. The work of the Eindhoven group in
supramolecular H-bonded conjugated organic materials has
been recently reviewed.103,104
Feature Article
Improved charge carrier mobility was demonstrated in
another recent study of solution-processable supramolecular
assemblies.105 Perylene bisimides bearing melamine sidegroups were dissolved in organic solvents by mixing with
complementary H-bonding barbiturate or cyanurate.
Spin-coating these solutions affords highly organized lamellar
assemblies with an electron eld-effect mobility up to 1 104 cm2 V1 s1 (Fig. 10). This work is particularly interesting
for the reliance of multiple components to achieve
Fig. 10 Perylene bisimide conjugated dyes functionalized with melamine
segments, providing self-assembly to yield H-bonded self-organized “tapes”
allowing electron-transport through stacked perylene bisimide moieties. Reproduced with permission from ref. 105.
Fig. 9 (a) Complementary H-bonded self-assembly of a hole-conducting conjugated molecule oligo(p-phenylenevinylene) and an electron-transporting one, perylene
bisimide, resulting in an ambipolar-transporting supramolecular material, from ref. 101. (b) Blue, green, and red fluorophores functionalized with complementary Hbonding end groups. (c) H-bonding mediated self-assembly of fluorophores, resulting in an efficient energy cascade to produce white light emission, (d and e).
Reproduced with permission from ref. 102.
3750 | J. Mater. Chem. B, 2013, 1, 3742–3753
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Feature Article
coassemblies, highlighting the large degree of tunability that
can be accessed by using H-bonding for supramolecular
assemblies.
Conclusions
Since Pauling's recognition of the crucial role that H-bonding
plays in biological materials, such as proteins, it has become
clear that H-bonding is an extremely important noncovalent
interaction ubiquitous in nature and remarkable for self-organization of complex molecular systems. H-bonding is likewise
found in natural dyes and pigments and has been explored in
the crystal engineering of industrially important pigments. Only
recently research has been started to elucidate the role that Hbonding can play in ionic and electronic conducting materials:
natural systems such as DNA and melanin have shown promise
that they can be used as electronic materials directly. H-bonded
pigments, many of which are of natural origin, such as indigo,
have proven recently to be highly promising organic semiconductors, outperforming many established materials especially in terms of operational stability. Signicantly, many of
these molecules are already known to be nontoxic and biodegradable. Finally, electronic and ionic-conducting conjugated
building blocks can be functionalized with H-bonding moieties
that accomplish self-assembly into useful supramolecular
structures. All three of these research directions converge on the
principle of employing H-bonds to control ordering in organic
solids. Since exactly these types of ordering interactions are
prevalent throughout biological systems, realization of organic
electronics based on H-bonded materials represents a major
step in the direction of biointegrated electronics.
We wish to warmly thank Dr Uwe Monkowius of the JKU for
valuable discussions and collaboration on X-ray diffraction
experiments. Support of the Austrian Science Fund (FWF)
within the Wittgenstein Prize and the ERC within the Advanced
Investigators Grant “So-Map” is gratefully acknowledged.
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